Oxygen generation electrode and oxygen generation apparatus

ABSTRACT

An oxygen generation electrode includes: a conductive substrate; and an oxide film formed on a first surface of the conductive substrate and containing Ba, Sn, and La or Sb, wherein the oxide film has a first absorption edge in a visible light region and a second absorption edge in an infrared light region.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of International Application PCT/JP2018/002092 filed on Jan. 24, 2018 and designated the U.S., the entire contents of which are incorporated herein by reference. The International Application PCT/JP2018/002092 is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2017-060981, filed on Mar. 27, 2017, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to oxygen generation electrodes and oxygen generation apparatuses.

BACKGROUND

Technology of generating an oxygen gas by decomposing water has been studied. A decomposition reaction of water is constituted by a combination of the following half reactions, and the oxygen gas is generated by the latter half reaction (Formula 2). Various propositions have been provided for oxygen generation electrodes suitable for the latter half reaction (Formula 2).

2H₂O+2e ⁻→H₂+2OH⁻  (Formula 1)

4OH⁻+4h ⁺→2H₂O+O₂  (Formula 2)

However, it has been difficult to generate an oxygen gas efficiently with an oxygen generation electrode.

Examples of the related art include International Publication Pamphlet No. WO 2012/137240 and Japanese Laid-open Patent Publication No. 2005-126295.

SUMMARY

According to an aspect of the embodiments, an oxygen generation electrode includes: a conductive substrate; and an oxide film formed on a first surface of the conductive substrate and containing Ba, Sn, and La or Sb, wherein the oxide film has a first absorption edge in a visible light region and a second absorption edge in an infrared light region.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are diagrams illustrating an oxygen generation electrode according to a first embodiment;

FIG. 2 is a diagram illustrating a configuration of an oxygen generation apparatus according to a second embodiment;

FIGS. 3A and 3B are a diagram illustrating an oxygen generation apparatus used for an experiment and a diagram illustrating a result of the experiment;

FIG. 4 is a diagram illustrating a characteristic of the oxygen generation apparatus according to the second embodiment;

FIGS. 5A and 5B are diagrams illustrating a modification example of the first embodiment; and

FIGS. 6A and 6B are diagrams illustrating a change in the composition of a BaLaSnO film according to the progress of etching.

DESCRIPTION OF EMBODIMENTS

An object of the embodiments is to provide an oxygen generation electrode and an oxygen generation apparatus capable of generating an oxygen gas efficiently.

According to an aspect, an appropriate conductive substrate and an appropriate oxide film are contained, and therefore an oxygen gas may be generated efficiently.

Hereinafter, the embodiments will be described in detail with reference to accompanying drawings.

First Embodiment

First, a first embodiment will be described below. The first embodiment is an example of an oxygen generation electrode. FIG. 1A is a section view of an oxygen generation electrode according to the first embodiment illustrating a configuration thereof.

As illustrated in FIG. 1A, an oxygen generation electrode 10 according to the first embodiment includes a conductive substrate 11, an oxide film 12 formed on a first surface of the conductive substrate 11 and containing Ba, Sn, and La or Sb, and an electrode 13 formed on a second surface of the conductive substrate 11. FIG. 18 is a diagram illustrating an example of absorption spectra of the oxide film 12. As illustrated in FIG. 1B, the oxide film 12 has a first absorption edge AE1 in a visible light region and a second absorption edge AE2 in an infrared light region.

Since the oxide film 12 in the oxygen generation electrode 10 according to the first embodiment has the first absorption edge AE1 and the second absorption edge AE2, water may be strongly photochemically oxidized. Therefore, according to the present embodiment, an oxygen gas may be generated efficiently.

For example, the conductive substrate 11 is an SrTiO₃ substrate doped with an n-type impurity such as Nb. For example, the concentration of the n-type impurity is 0.5% by mass to 2.0% by mass. The thickness of the conductive substrate 11 is, for example, 0.1 mm to 1.0 mm. For example, the chemical formula of the oxide contained in the oxide film 12 is represented by Ba_(x)La_(y)Sn_(z)O_(3-δ) or Ba_(x)Sb_(y)Sn_(z)O_(3-δ). The sum of x, y, and z is 2, but the composition changes to 0.5<(x+y)/z<1 after etching that will be described later. For example, b is 0 or more and less than 0.8. The thickness of the oxide film 12 is, for example, 50 nm to 150 nm. The electrode 13 is, for example, an Au film having a thickness of 10 nm to 100 nm.

In an example, the conductive substrate 11 is an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, the oxide film 12 is a Ba_(0.95)La_(0.05)SnO₃ film having a thickness of 80 nm, and the electrode 13 is an Au film having a thickness of 50 nm. FIG. 1B illustrates absorption spectra of this example. The energy of the first absorption edge AE1 is about 3 eV, and the energy of the second absorption edge AE2 is about 1 eV. In another example, the conductive substrate 11 is an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, the oxide film 12 is a Ba_(0.90)La_(0.10)SnO₃ film having a thickness of 100 nm, and the electrode 13 is an Au film having a thickness of 50 nm. In yet another example, the conductive substrate 11 is an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, the oxide film 12 is a Ba_(0.95)Sb_(0.05)SnO₃ film having a thickness of 100 nm, and the electrode 13 is an Au film having a thickness of 50 nm.

Next, an example of a method for manufacturing the oxygen generation electrode 10 according to the first embodiment will be described. In this example, first, the oxide film 12 is formed on the first surface of the conductive substrate 11 by a pulsed laser deposition (PLD) method. Next, the electrode 13 is deposited on the second surface of the conductive substrate 11.

Second Embodiment

Next, a second embodiment will be described. The second embodiment relates to an oxygen generation apparatus including the oxygen generation electrode 10. FIG. 2 is a diagram illustrating a configuration of the oxygen generation apparatus according to the second embodiment.

As illustrated in FIG. 2, the oxygen generation apparatus 20 according to the second embodiment includes an aqueous electrolyte solution 22 contained in a tank 21, the oxygen generation electrode 10, a reference electrode 23, and a counter electrode 24 that are provided in the aqueous electrolyte solution 22, and a potentiostat 25 connected to the oxygen generation electrode 10, the reference electrode 23, and the counter electrode 24. For example, the reference electrode 23 is an Ag/AgCl electrode, the counter electrode 24 is a Pt electrode, and the aqueous electrolyte solution 22 is a 0.05 M to 0.5 M KOH aqueous solution.

In the oxygen generation apparatus 20, the oxygen generation electrode 10 is used as a working electrode. Therefore, by adjusting the potential of the oxygen generation electrode 10 with respect to the reference electrode 23, an oxygen gas may be efficiently generated.

Here, experiments that the present inventors conducted will be described. In this experiment, an oxygen generation apparatus 120 illustrated in FIG. 3A was used. The oxygen generation apparatus 120 included an oxygen generation electrode 110 including an SrTiO₃ substrate 111 having a thickness of 0.5 mm and doped with 1% by mass of Nb, a Ba_(0.95)La_(0.05)SnO₃ film 112 having a thickness of 80 nm, and an Au electrode 113 having a thickness of 50 nm. The oxygen generation electrode 110 was disposed at the bottom of a tank 121, a 0.1 M KOH aqueous solution 122 was charged in the tank 121, an Ag/AgCl electrode 123 and a Pt electrode 124 were disposed in the KOH aqueous solution 122, and the Au electrode 113 serving as a working electrode, the Ag/AgCl electrode 123 serving as a reference electrode, and the Pt electrode 124 serving as a counter electrode were connected to a potentiostat 125.

Then, a current that flowed when irradiated with sunlight having an illuminance of 598 mW/cm² was measured by using a solar simulator. The result of this is illustrated in FIG. 3B. As illustrated in FIG. 3B, in this experiment, on/off was switched repeatedly at short intervals since the start until the elapse of 400 seconds, and an on state was maintained for a relatively long period since the elapse of 500 seconds until the elapse of 900 seconds. An excellent current of about 0.7 mA/cm−2 was obtained in both of the case where the time of the on state was short and the case where the time of the on state was long. This means that an oxygen gas may be generated efficiently by the oxygen generation apparatus 120.

FIG. 4 is a diagram illustrating a characteristic of the oxygen generation apparatus 20 according to the second embodiment. FIG. 4 also illustrates, for comparison, characteristics of two comparative examples. In the second embodiment (solid line), the oxygen generation electrode includes an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, a Ba_(0.95)La_(0.05)SnO₃ film having a thickness of 80 nm, and an Au film having a thickness of 50 nm.

In a first comparative example (broken line), the oxygen generation electrode includes an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, and an Au film having a thickness of 50 nm.

In a second comparative example (two-dot chain line), the oxygen generation electrode includes an SrTiO₃ substrate having a thickness of 0.5 mm and doped with 1% by mass of Nb, and Au nanopartides. As illustrated in FIG. 4, according to the second embodiment, a conversion efficiency much higher than the first and second comparative examples may be achieved.

As illustrated in FIG. 5A, the oxide film 12 may be etched into island shapes. By etching the oxide film 12 into island shapes, a higher conversion efficiency may be achieved in some cases. FIG. 58 is a diagram illustrating an example of a relationship between the coverage of the oxide film 12 on the substrate and the conversion efficiency. In this example, the oxide film 12 is a Ba_(0.95)La_(0.05)SnO₃ film. In the case where the coverage was 90%, a higher conversion efficiency than in the case of 100% was achieved for both of light of 525 nm corresponding to green and light of 475 nm corresponding to blue. As illustrated in FIG. 5B, the coverage is preferably 80% or more, for example, 85% to 95%.

FIGS. 6A and 6B are diagrams illustrating a change in the composition of a Ba_(0.95)La_(0.05)SnO₃ film according to the progress of etching.

FIG. 6A illustrates a result of X-ray photoelectron spectroscopy (XPS) of the 3 d orbital of Ba, and FIG. 68 illustrates an XPS measurement result of the 3 d orbital of Sn. In FIGS. 6A and 6B, broken lines indicate measurement results of the Ba_(0.95)La_(0.05)SnO₃ film deposited to a thickness of 60 nm, and solid lines indicate measurement results of the Ba_(0.95)La_(0.05)SnO₃ film etched to a thickness of 10 nm after deposition. As illustrated in FIGS. 6A and 6B, whereas the ratio of Ba atoms to Sn atoms (Ba/Sn ratio) is about 0.97 before the etching, the Ba/Sn ratio is about 0.61 after the etching. This indicates that the etching makes the ratio of Sn atoms higher and the ratio of Ba atoms lower. In the chemical formula of Ba_(x)La_(y)Sn_(z)O_(3-δy), 0.5<(x+y)/z<1 is satisfied. The sizes of island-shaped portions of the etched oxide film 12 are preferably in the order of nanometer.

In the case where a current is capable of being directly supplied to the conductive substrate 11, the electrode 13 does not have to be provided.

All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. An oxygen generation electrode comprising: a conductive substrate; and an oxide film formed on a first surface of the conductive substrate and containing Ba, Sn, and La or Sb, wherein the oxide film has a first absorption edge in a visible light region and a second absorption edge in an infrared light region.
 2. The oxygen generation electrode according to claim 1, wherein the oxide film contains an oxide represented by a chemical formula of Ba_(x)La_(y)Sn_(z)O_(3-δ) or Ba_(x)Sb_(y)Sn_(z)O_(3-δ) (0.5<(x+y)/z<1).
 3. The oxygen generation electrode according to claim 2, wherein 6 satisfies 0≤δ<0.8.
 4. The oxygen generation electrode according to claim 1, wherein a coverage of the oxide film on the conductive substrate is 80% to 100%.
 5. The oxygen generation electrode according to claim 1, wherein the conductive substrate contains an oxide doped with an n-type impurity.
 6. The oxygen generation electrode according to claim 5, wherein the oxide is SrTiO₃.
 7. The oxygen generation electrode according to claim 1, wherein an electrode is formed on a second surface of the conductive substrate.
 8. The oxygen generation electrode according to claim 7, wherein the electrode contains Au.
 9. An oxygen generation apparatus comprising: an aqueous electrolyte solution; the oxygen generation electrode according to claim 1, the oxygen generation electrode being provided to be immersed in the aqueous electrolyte solution; a reference electrode and a counter electrode provided to be immersed in the aqueous electrolyte solution; and a potentiostat connected to the oxygen generation electrode, the reference electrode, and the counter electrode. 